In the field of LED drivers for offline applications such as retrofit lamps, solutions are demanded to cope with high efficiency, high power density, long lifetime, high power factor and low cost, among other relevant features. While practically all existing solutions compromise one or the other requirement, it is essential that the proposed driver circuits properly condition the form of the mains power into the form required by the LEDs while keeping compliance with present and future power mains regulations. Of critical importance is to guarantee a minimum perceptible light flicker (preferably zero)
Typically, two series connected power stages are employed to obtain high power factor while keeping the output power constant throughout a mains cycle (or supply cycle, i.e. the cycle of the mains voltage or the supply voltage). Also known are converters with a single power converting stage that allow high power factors (HPF) by means of integrating a boost converter operating in discontinuous conduction mode. These converters actually combine two power conversion stages.
For some retrofit LED driver products compatibility with phase-cut dimmers is desired, which however were designed to drive much larger loads including resistive and/or reactive components, in particular R, RL, C, or RLC components. Many of those dimmers are of two-wire type.
Enabling maximum dimmer compatibility for low power LED lamp modules (also called LED units hereinafter) involves several problems, which mainly have to do with the power rating mismatch between lamp and dimmer. Such problems include, for instance, to provide sufficient holding currents for thyristors or triacs used in the dimmers, or to deal with unavoidable oscillations between LF filter elements of typical dimmer circuits, which may lead to uncontrolled flicker.
Existing solutions—if available at all—typically show increased losses, such as known bleeder circuits often used in such driver devices (see e.g. JP 2010-2122678), and also require several passive and/or semiconductor components.